Why Does It Flood When Levees Don’t Break? And Other Questions. Part 1


Bigger Pie Forum | Why Does It Flood When Levees Don’t Break? And Other Questions. Part 1 | Kelley Williams
Q.  What is a Mississippi River Flood?
A.  An event that causes the river to cover land not normally under water.  Levee breaks cause floods.  There hasn’t been a main line levee break since 1937.  But it floods anyway — when the river gets higher than the land between its channel and the levees and hills that contain it (the batture). And when the river gets higher than the Yazoo and other tributaries that flow into it and causes them to back up (backwater flood).
Q.  How big are batture and backwater floods?
A.  Two million acres in Mississippi and Louisiana flooded in 2011 and caused $3 billion estimated damage.  Thousands of acres in Mississippi flood every year. Its batture contains 600,000 acres. It is 400 miles long and up to seven miles wide between the river and the hills below Natchez.  The Yazoo, Big Black, and Homochitto Rivers have large drainage basins that have backwater floods.
Q.  How are floods measured?
A.  Flood metrics include height, flow, duration, frequency, timing (when they occur) and damage. Height, duration, frequency and timing are observed.  Damage is estimated.  Flow is calculated.  Generally, the greater the flow, the higher the flood.  The 1927 flood had the greatest flow, but wasn’t the highest because levees broke and released it.
Q.  Where are floods measured?
A.   River height or stage is measured at river towns like Greenville, Vicksburg, Natchez and at other locations.  Each location has its own gage and “flood stage” not correlated with elevation or other gages.  For example, flood stage at Vicksburg is 43 feet, 48 feet at Natchez, and 35 feet at Baton Rouge.  The highest stage ever measured at Natchez was 62 feet in 2011, and the lowest was minus 2 feet

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